The present disclosure relates to a dynamic mode atomic force microscope (AFM). The disclosure further relates to a method of calibrating a force in a dynamic mode AFM, determining the force in a dynamic mode AFM measurement, and using a dynamic mode AFM for applying a predetermined force to a sample surface.
In a dynamic mode AFM (e.g. Tapping Mode AFM or Non-Contact mode AFM), an AFM tip periodically approaches, interacts and retracts from a sample surface and experiences for example long range attractive forces and/or short range repulsive forces. The contact area between the tip and sample surface is typically in the nanoscale range which implies that the tip-sample forces, in the neighborhood of nano Newtons, can create huge stress and easily damage the sample surface or the tip itself. Accordingly, it is desired to determine the time varying nano-mechanical forces during imaging for sensitive and fragile samples and at the same time for increasing the tip lifetime in tapping mode AFM which is the preferred mode of operation.
In a dynamic mode AFM, the frequency, amplitude and phase of the cantilever oscillation are the primary observable parameters of the system. For example, a sine wave is applied on an oscillator element and the cantilever is driven into oscillation at the same frequency with the oscillator, a laser beam is directed onto the cantilever and a reflection of the laser beam is monitored to determine the oscillation frequency, amplitude and phase. However, conventionally, the nonlinear interaction forces between the probe tip and sample surface cannot be extracted from the observable parameters of the sinusoidal signal; only energy dissipation can be monitored. See for example J. P. Cleveland et al., Appl. Phys. Lett., Vol. 72, No. 20, 1998.
To determine the tip sample interactions various method exist. For example, F. L. Degertekin et al. (Rev. of Sci. Instr. Vol. 77, 2006) describes a micro-machined membrane with an integrated displacement sensor to extract tip sample interactions. As another example, O. Sahin et al. (Nature Nanotechnology, Vol. 2, 2007) describes a torsional harmonic cantilever wherein the structure of the cantilever is modified to place the tip that is offset from the long axis of the cantilever. Torsional motion of the cantilever is used to extract tip sample interactions. As another example, A. F. Sarioglu et al. (Journal of Microelectromechanical Systems, Vol. 20, 2011) describes an integrated high bandwidth force sensor, wherein the cantilever has an interferometric force sensor to resolve tip sample interactions. Diffraction grating at the end of the cantilever beam is used as a force sensor to extract tip sample interactions. Unfortunately, the known methods require custom construction of micro-machined elements and/or adaptation of the probe shape.
Accordingly it is desired to provide methods and systems for determining force in a dynamic mode AFM wherein disadvantages of the prior art are alleviated. For example, it is desired to measure tip sample interaction for all types of AFM cantilevers with different geometries such as triangular, rectangular, or special design. Furthermore it is desired to measure the tip sample interaction for all kinds of operation modes at static, dynamic or quasi-static regimes. Furthermore it is desired to monitor the change in tip sample interactions for different surface scenarios.